Journal of African Earth Sciences, Vol. 7, No. 3, pp. 553-559, 1988 (1731-7247/88 $3.00 + 0.00 Printed in Great Britain © 1988 Pergamon Press plc

Beach morphodynamics along the Nigerian coastline: implications for coastal engineering projects

EFFIOM EDEM ANTIA* a n d EYO ETIM NYONG-~

*Department of Geological Oceanography, Institute of Oceanography, University of Calabar, Calabar, Nigeria

tDepartment of Geology, University of Calabar, Calabar, Nigeria

( Received ]or publication 5 May 1987)

Abstract--The morphodynamic states of five Nigerian beaches differing in topographic, tidal and grain size characteristics have been examined as a basis for suggesting the modality for efficient and cost-effective implementation of sand replenishment on eroding beaches. Three of the beaches, namely Forcados, Brass and Ibeno, fluctuated between the dissipative and intermediate morphodynamic states while Badagry and Victoria Beaches varied between reflective and intermediate states. Recent and on-going investigations at Ibeno Beach reveal that reflectivity values approaching those of dissipative state were characterized by low beach mobility and net sediment exchange minimal. On the contrary, a shift towards reflective state with low reflectivity values resulted in high and rapid erosion. The major implication of the above results demands that sand replenishment must be implemented in a manner that will ensure the persistence of dissipative condition. Given the present state of knowledge of wave conditions along the Nigerian coast, dissipative state would be enhanced if sand replenishment does not increase the foreshore and surf zone slope beyond 7 °.

INTRODUCTION

WITH THE exception of some coastline segments con- tiguous to estuaries and a few others where engineering stability measures have been employed, beach erosion is a widespread phenomenon along Nigeria's 800 km long sandy coastline (Ibe and Antia, 1983; Ibe etal. 1984). As highlighted in the above reports, coastal settlements, recreational grounds, ports and oil export handling facilities as well as industrial and military infrastructures are some of the erosion-threatened projects located along the coast and requiring protection.

Research attention so far given to evaluate the mag- nitude and mode of beach changes along the different sectors of the coastline by our coastal scientists and engineers is grossly inadequate when compared with the severity of the problem. It is reckoned by Ibe and Antia (1983) that the annual average of beach erosion in the Lagos area is in the 20-30 m range. The persistence of such scale of erosion, considered to be one of the highest anywhere in the world, would bring about economic and social disaster for the country.

Besides the afore-mentioned, some pertinent infor- mation on the erosion phenomenon along the Nigerian coastline have been presented by Usoro (1971, 1977); Fawora Assoc. (1981); Civtra Consultants (1982); Oyegoke (1983); Antia (1985, 1986); Antia and Nyong (1986) and Nyong and Antia (1986).

In principle, several coastal engineering measures exist for alleviating beach erosion. These include in the main, sand replenishment and deployment of structural units such as revetments, bulkheads and sea walls all of which are parallel to the shore. Other structures such as groynes and jetties are usually perpendicular to the shore. However, largely because of its comparatively lower capital and maintenance cost and the preservation of the aesthetic and recreational values of beaches, sand

replenishment has been the most frequently employed measure to control coastal erosion in Nigeria.

In view of the anticipated large-scale sand replenish- ment projects to be embarked upon by the Federal Government along a number of eroding beaches, our research efforts were mainly directed to ensuring the rational implementation of the afore-mentioned coastal protection strategy. Indeed, the generally poor perfor- mance of sand replenishment projects and other engineering structures to stabilize beaches world-wide emanates from the non-cognizance of the distinct peculiarities of beaches such as their morphology, surf zone dynamics as well as wave-current interactions.

The above gaps in information have been mainly bridged by the morphodynamic classification of beaches (Fig. 1) recently put forward by Wright and Short (1983).

This study specifically examined the morphodynamic states of five different beaches on the Nigerian coast (Fig. 2) as a basis for predicting the performance of sand replenishment projects on such and similar beaches.

MORPHODYNAMIC STATE OF BEACHES INVESTIGATED

The study sites as shown in Fig. 2 encompass various segments of the following five beaches: Badagry, Vic- toria, Forcados, Brass and Ibeno. The beaches have comparable wind pattern, and moderate to high wave energy levels. The topographic and tidal conditions however vary.

The first two beaches are located within a microtidal (0-2 m tidal range) environment. Grain sizes are medium to coarse (0.55-0.61 mm) while beach face is steep (8-22°). The other three are mesotidal (2--4 m tidal range) beaches; they are largely fine-grained (0.18- 0.34 mm) and are of low gradient (<8°).

553

554 E r r ] o M Et)EM ANTIA a n d E ~ o ETIM NYONG

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Beach morphodynamics along the Nigerian coastline: implications for coastal engineering projects 555

VICTORIA BADAGRY BEACH BEACH

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Fig. 2. Outline of the Nigerian coastline showing studied beaches.

At present, data on wave conditions and morphologic change are most intensive for the Ibeno Beach. Repor ts of Ibe and Antia (1983) and Ibe et al. (1984) constitute the major source of information for the other beaches, excluding Badagry. Investigations on the latter beach was made by the first author at different times in 1984 and 1985.

The three major beach states shown in Fig. 1 were distinguished on the basis of the surf-scaling or reflec- tivity paramete r (e) of Inman and Guza (1982). The reflective beach state denotes those having values of e < 2.5 while e > 33 defines the dissipative state.

The equation for obtaining e is given thus:

e - -

g(tan ~0) 2

where H is breaker height (m), T is period (secs), g is acceleration due to gravity (10 m S -2) and q~ is beach/surf zone gradient in degrees.

In the study, breaker height was estimated with the aid of a graduated staff held in position close to breaker

point. Wave period was determined with the aid of a stop watch while slope of beach/surf zone was obtained using the modified Emery ' s (1961) profiling technique. Profiles were made between established landward marker seawards to permissible water depth.

Apar t f rom providing information on beach slope, repetitive beach profiles further provided insights as to the temporal and spatial mobility of beach segments and the magnitude of sediment exchange. Knowledge of these factors is crucial in assessing the performance of sand replenishment projects made on different mor- phodynamic beach states.

Because of the relative consistency of acquired data on beach changes at Ibeno (on a fortnightly basis), discussion on the implementat ion strategies of sand replenishment at other beaches are extrapolated based on the results obtained f rom the former. In so far as the variability in beach state constitutes the sole criterion for evaluating performance of sand replenishment, no seri- ous shortcoming was envisaged in the above extrapola- tion.

The data set of beach states for the different beaches as defined by e values are shown in Table 1. Table 2

Table 1. Reflectivity values for the different beaches at various time intervals.

Forcados Beach Brass Beach Ibeno Beach Victoria Beach

April 1983 May 1983 July 1983 August 1983 November 1983 January 1984

e e e 6.27 February 1983 11.82 February 1983 9.95 March 1983 9.31 April 1983 11.80 April 1983 12.95 April 1983

47.01 July 1983 34.05 May 1983 32.71 June 1983 24.01 August 1983 28.21 July 1983 24.24 August 1983 3.57 September 1983 27.08 August 1983 19.37 November 1983

10.44 November 1983 34.50 October 1983 17.63 December 1983 January 1984 5.71 January 1984 5.97

e

0.54-2.78 1.18

0.63-4.16 1.54-2.02 2.43-5.01 1.97-4.11

5 5 6 E F F I O M E D E M A N T I A a n d E Y o E T I M N Y O N G

Tab le 2. B e a c h mobi l i ty in r e l a t i on to ref lect iv i ty p a r a m e t e r s o b t a i n e d for I b e n o Beach s ta t ions b e t w e e n A u g u s t 1985 and June 1986

Al BI CI DI El II GI HI

R

e D e D e D e D e D e D e D e D 15.24 -1,55 8 31 -13.78 5 50 -111.31 0 92 +2.43 17 89 +1.12 I0 69 -20.72 5.40 +35 16 2 21 -10.90 4.70 -7.71 9.87 -2 .30 3.69 -0.1 15.95 +11.74 5.87 -0.40 7,23 +1.65 32.22 -4.81 18.70 -0.55 6.29 -7 .89 7.29 +0.60 4.98 -1 .4 6.02 +1.99 40.91 +0.5 16.74 -1 50 18.37 - 0 70 11.73 -6 .80

11.43 -5.05 8.46 -0.30 6.30 +0.2 5,18 -1 .9 13.81 -5.22 12.69 -3.20 6.92 -3 .30 19.55 -1.30 7.41 - 6 4 33.72 -1.1 4.56 -6 .07 7.69 -0.82 5.13 -1.62 17.06 -0 .30 9.95 +0.25 1.03 -3.911

10.71 +0.90 3.72 +0.40 3.91 -3.38 15.36 -1 .8 8.38 -0 .7 44.58 +0,60 10.02 -11.85 6.75 +7.511 18.14 0.00 8.74 -6.68 7.61 -5 .7 28.93 +0.30 21.23 -4 .80 46.34 +2.4 111.31 -26.30 5.38 -4.911 31.77 +0.7 3.87 - 0 15 10.90 0.00 8.55 -2.38 9.67 -7.46 16.72 -11.50 14.91 +0.25

0.87 0.46 0.45 0 52 0.49 0.45 0.47 0.40

e = reflectivity parameter. D = beach change ( - erosion; + accretion), R = coefficient of multiple correlation.

provides information on beach mobility and corre- sponding e values for eight stations, AI-HI (Fig. 2), at Ibeno Beach. The data were acquired between August 1985 and June 1986. Plots for stations AI, FI and GI

shown in Fig. 3 illustrate the general trend for the studied beach of Ibeno. Figure 4 shows the relationship between volumetric beach change for both instantane- ous and antecedent values of e obtained at various times

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Beach morphodynamics along the Nigerian coastline: implications for coastal engineering projects

m 5/m

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from the different segments of the Ibeno Beach. The latter provide an insight as to the variability of beach profiles and hence rapidity of sediment exchange.

RESULTS AND DISCUSSION

The reflectivity parameter data shown in Table 1 indicate that the beaches of Forcados, Brass and Ibeno fluctuated between the dissipative and intermediate states with the latter being predominant. In contrast, Victoria Beach exhibited variation between reflective and intermediate states, with the former being preva- lent. Similar mode and range in beach state was found at Badagry Beach, where the reflectivity parameter varied between 0.68 and 4.06.

Field observations of widely varying beach charac- teristics in Australia documented by Wright and Short (1983) indicate that the dissipative beach state was the most stable while temporal variability in beach mobility was greatest on the intermediate beach state. Addition- ally, the authors noted that except in peculiar cir- cumstances where wave energy reaching shore was con- siderably filtered, reflective conditions were transient.

The non-occurrence of reflective conditions (e < 2.5) at the beaches of Forcados, Brass and Ibeno (Table 1) is in agreement with the above reported observations. However the same is not the case for the beaches of Victoria and Badagry where modally reflective condition occurred. While the persistence of the reflective state of the study stations of Victoria Beach can be attributed in part, to the sheltering effect of the moles constructed at

558 EFFIOM EDEM ANTIA and Evo ETIM NYONG

the Lagos Harbour and to the executed sand replenish- ment, one cannot account with certainty for the persis- tence of similar condition at Badagry.

It is however suggested that the dominance of the reflective condition at the latter might be a result of the coarse-grained sediments comprising the beach rather than a manifestation of a fully accreted beach which Wright and Short (1983) note is often reflective, although generally short-lived. However , it is also known that steep beach slopes generally associated with coarse sediments commonly tend to exhibit reflective signatures. The authors further note that the reflective beach state is the most vulnerable to erosion under rising wave energy. This is primarily because of the absence of subaqueous sand storage and the ease with which accen- tuated wave run-up occurs under long period and low steepness swells. The run-up greatly facilitates the pro- cesses of beach face scarping and berm cutting. The low beach mobility of the dissipative state on the other hand is largely attributed to the high degree of attenuation of the shoaling waves, this being a consequence of its low gradient and characteristic sub-aqueous sand storage.

In contrast with the reflective state, surf zone proces- ses on dissipative beaches less easily attain destructive amplitudes enough to cause appreciable changes on the beach. Beach change under dissipative state is com- monly associated with very high wave energy conditions in which case bores of partially dissipated waves may induce beach retreat by scarping at the berm base. Short and Hesp (1982) consider such beach retreat to proceed without a net offshore loss of beach material.

The very high mobility and erosion vulnerability of the intermediate beach state results from the complexity of the interaction between surf zone processes and the array of dynamic bar forms typical of such state. In addition to berm cutting from accentuated run-up and bore-induced scarping, erosion at this beach state also progresses through scouring of beach by the typically persistent rip current circulation (Wright and Short, 1983).

The relationship between both beach mobility and net volumetric change versus reflectivity parameter based, in part, on data of Antia and Nyong (1986) and shown in Figs 3 and 4 do not reveal a simple linear correlation. For the beach mobility plot, coefficient of multiple correla- tion ranged between 0.87 at station AI to a minimum of 0.40 at HI. Examination of the distribution pattern of the data points for the different stations indicate that low values of e were generally associated with higher mag- nitude of negative beach mobility (or erosion). Higher values of e tended to be associated with stable or accre- tional beaches.

Although as previously noted, modally, intermediate beach state prevailed on Ibeno Beach, however some elements of dissipative and reflective beach states are incorporated into the intermediate state for high and low values of e respectively. The high temporal variability in beach mobility of the intermediate state is well illus- trated at station GI where extremities in beach accretion and erosion occurred. The accretion of 35.16 m at the

above station observed on 17 August 1985 was due to the welding of a subaqueous sand bar to the berm. As a consequence of continuous berm cutting, beach scarping and perhaps rip-current induced scouring, a substantial amount (26.30 m) of the welded bar had become eroded by 24 May 1986. Generally, given the fornightly interval of survey, erosion was mostly below 10 m while accretion was typically under 5 m.

In Fig. 4, two nearly identical plots of net volumetric change (m3/m) in beach profiles versus instantaneous and antecedent reflectivity parameter have been pre- sented. Clustering of data points in both cases seem to suggest a higher variability of sediment exchange and sediment loss from the profiles of beaches with low values of e. The decrease in sediment loss from the profile and reduction in profile variability with increase in reflectivity parameter as noted on this beach (Antia and Nyong, 1986) corroborate the reported observations of Wright and Short (1983). The higher coefficient of multiple correlation of 0.2 obtained with the antecedent reflectivity parameter compared with the instantaneous one of 0.1 is in conformity with the above authors' earlier observation that pattern of beach change was profoundly influenced by the former.

The rather low coefficients of correlation obtained above are probably as a result of data treatment. In order to make for easy comparison, net volumetric changes were normalised by dividing with varying profile lengths. Apart from normalization, a major problem encountered in field sediment budgeting of beaches from repetitive profiles as noted by Antia (1986) is defining the offshore limit of minimal sediment exchange due to surf zone processes. The above, often referred to as closure depth of profile, is a major source of error in both subaerial and subaqueous profiling of beaches.

Despite these shortcomings, the results obtained are sufficient for making comments on the effectiveness of the present sand replenishment projects in Nigeria. Dean (1983) and a host of other coastal engineering literature provide general guidelines for beach nourish- ment such as suitability of materials, volume and other design characteristics. However pertinent to this discus- sion is the establishment of stable equilibrium profiles on the basis of the known wave conditions and surf zone processes for the beach in question.

From the foregoing results and discussion, it is evident that the dissipative beach state would be the most stable temporally and also one in which permanent loss of sediments from the beach to offshore is minimal. Thus the practice such as at the Victoria Beach where sedi- ments were dredged from the nearshore and piled to form protective barrier is despicable and should be discontinued for a number of reasons.

Firstly, nearshore sand dredging, by deepening the region, diminishes the attenuation of incident waves. The consequence of which is that waves can generally attain higher heights and break much closer to shore. Both conditions facilitate sediment mobility on beaches. Also associated with the steepening of the nearshore profile is the establishment of reflective conditions where

Beach morphodynamics along the Nigerian coastline: implications for coastal engineering projects 559

large-scale erosion following wave run-up is enhanced. However dredging should be permitted only where this is done along trenches parallel to the shore and at depths much beyond that of active wave action. Additionally, dredged materials should not be arbitrarily placed on the beach or beach sectors being restored. Rather, they should be placed in a manner that ensures fair replication of the natural profile. Secondly, given the present sand replenishment approach, the maintenance costs of such projects are bound to escalate because of their short life span. Thirdly, there could be a possible destruction of the beauty and aesthetic appeal of beaches where replenishment is indiscriminately effected. The present authors share the opinion of earlier workers like Usoro (1971) and Ibe and Antia (1983) on the Victoria Beach that it is preferable and more economical on the long-run to replenish the eroding Victoria Beach with sediments trapped by the Lagos Harbour moles. However for other segments of the coastline, suitable materials can be found in abundance by dredging the nearby estuaries.

Furthermore, as evident around the Lagos coast, any engineering structure on the coast that is likely to obstruct the easterly littoral drift of sediment (northerly at the lower flanks of the Niger Delta) or enhance the deepening of the nearshore profile are most likely to aggravate erosional rates. Other factors such as dam- ming of rivers upland, global sea level rise as reviewed by Bird (1983) may also contribute to the problem to varying extents.

However, taking cognizance of the possible extreme wave conditions along the Nigerian coast, it is suggested that in order to ensure that replenished beaches are not brought to the realm of modally reflective morpho- dynamic beach state where erosional sensitivity is high and sediment exchange rapid, the replenished beach foreshore slopes should never exceed 7 ° .

CONCLUSION

The main implication of the undertaken investigation to beach nourishment, a common practice in coastal engineering, entails that implementation of replenished sand must proceed in such a manner that will ensure the prevalence of dissipative condition. The latter has the highest temporal stability and least net sediment exchange. On the other hand, any practice such as nearshore dredging, placement of replenished sediments or other forms of coastal structures likely to enhance reflective conditions should be discouraged given their high sensitivity to erosion.

Finally, considering the prevailing wave conditions off the Nigerian coast, modally reflective beach state will rarely occur on foreshore slope of 7 ° or less. The wave

energy on the few instances where reflective conditions may occur with the above slope condition will be too mild to produce any appreciable erosion.

Acknowledgements--Gratitude is expressed to Dr A. D. Short, Coas- tal Studies Unit, University of Sydney, Australia for encouragement and literature provided. EEA is highly indebted to Dr B. W. Flem- ming, Senckenberg Institute of Marine Geology, Wilhelmshaven, F.R.G. for his enthusiastic support during a research tenure at the latter. Usen Umana has been helpful in the field.

REFERENCES

Antia, E. E. 1985. Essence of sediment budget as a planning criterion in stability of shore lines. Nigerian Engineer (in press).

Antia, E. E. 1986. Areal variations in textural characteristics of sediments of the coastline of Cross River State. Contribution to the Research Bulletin of the Institute of Oceanography, University of Calabar, Nigeria (Unpublished).

Antia, E. E. and Nyong, E. E. 1986. Beach cusps, beach states and erosional susceptibility of Nigerian beaches. Paper presented at the First National Workshop on Oceanography, Institute of Oceanog- raphy, University of Calabar, Nigeria, 26-27 June 1986.

Bird, E. C. F. 1983. Factors influencing beach erosion and accretion: a global review. In: Sandy Beaches as Ecosystems (edited by McLachlan, A. and Erasmus, T.), pp. 709-717. Junk Publishers, The Hague.

Civtra International Consultants 1982. Feasibility studies and prelimi- nary design for anti-erosion measures along the national coastline of Odonla, Odofado, Jirinwo, Awoye and Ubalu within Ondo State. Report submitted to the Federal Ministry of Works.

Dean, R. G. 1983. Principles of beach nourishment. In: CRC Hand- book of Coastal Processes and Erosion (edited by Komar, P. D.), pp. 217-231. CRC Press, Boca Raton, FL.

Emery, K. O. 1961. A simple method of measuring beach profiles. Limnol. Oceanog. 6, 90--93.

Fawora, Rehnardet and Associates 1981. Stabilisation and preliminary engineering designs of Victoria Beach, Lagos. Reports submitted to the Federal Ministry of Works, 3 Vols.

Ibe, A. C. and Antia, E. E. 1983. Preliminary assessment of the impact of erosion along the Nigerian shoreline. Technical Paper No. 13, Nigerian Institute for Oceanography and Marine Research, Lagos.

Ibe, A. C., Awosika, L. F. and Antia, E. E. 1984. Coastal erosion research project. Progress Report No. 2, Nigerian Institute for Oceanography and Marine Research, Lagos.

Inman, D. L. and Guza, R. T. 1982. The origin of swash cusps on beaches. Mar. Geol. 49,133-148.

Nyong, E. E. and Antia, E. E. 1986. The applicability of a sediment- texture model to beach dynamics on dissipative beaches east of the Niger delta. Paper presented at the National Seminar on Flooding and Erosion along the Nigerian Coastline. University of Lagos, 22-23 May 1986.

Oyegoke, E. S., Osoba, E. B., Oladapo, O. O., Uzochukwu, B. N. and Ibrahim, A. 1983. Coastal erosion problems in Nigeria and some measures adopted over the years for their solution. Paper presented at the International Conference on Coastal and Port Engineering in Developing Countries, Colombo, Sri Lanka, 20-26 March 1983.

Short, A. D. and Hesp, P. A. 1982. Wave, beach and dune interactions in Southeastern Australia. Mar. Geol. 48,259-284.

Usoro, E. J. 1971. Recent rates of shoreline retreat at Victoria Beach, Lagos. Nigerian Geog. J. 14, 49-58.

Usoro, E. J. 1977. Coastal development in Lagos area. Ph.D. Thesis, University of Ibadan.

Wright, L. D. and Short, A. D. 1983. Morphodynamics of beaches and surf zones in Australia. In: CRC Handbook of Coastal Processes and Erosion (edited by Komar, P. D.), pp. 217-231. CRC Press, Boca Raton, FL.